1. Field of the Invention
This invention relates to a polarizing plate for absorbing one of mutually orthogonal polarized components (P-polarized wave and S-polarized wave) and transmitting the other, and a method for manufacturing the same.
2. Description of the Related Art
The image formation principle of liquid crystal display devices essentially requires a polarizing plate provided on a surface of a liquid crystal panel. The function of the polarizing plate is to absorb one of mutually orthogonal polarized components (P-polarized wave and S-polarized wave) and to transmit the other.
Dichroic polarizing plates containing iodine or dye type organic polymer materials in films have been widely used as this kind of polarizing plate. These polarizing plates are generally manufactured by using a method including steps of dyeing a polyvinyl alcohol film with dichroic material such as iodine, performing crosslinking with a crosslinking agent and then performing uniaxial stretching. The dichroic polarizing plates tend to shrink since the manufacturing process includes this stretching step. In addition, because the polyvinyl alcohol film employs hydrophilic polymer, it is highly deformable especially under humidified conditions. Moreover, using a film as a base results in insufficient mechanical strength of the device, which may require a transparent protective film glued thereto.
In recent years, the liquid crystal display devices have been widely used in many purposes and the functionalities of them have been sophisticated. Accordingly, high reliability and durability are required on each device constituting the liquid crystal display devices. For example, in the case of liquid crystal display devices using a light source with a large luminous energy such as transmissive liquid crystal projectors, polarizing plates are exposed to intensive radiant rays. Consequently, the polarizing plates used for these are required to have an excellent heat-resisting property. However, since the above described film-based polarizing plates are made of organic materials, a limitation exists in enhancement of the property.
An inorganic polarizing plate having an excellent heat-resisting property is commercially available from Corning Incorporated in the USA under the trade name of POLARCOR™ (a highly durable borosilicate glass containing elongated silver crystals). This polarizing plate has a structure in which silver particles are diffused into glass and uses no organic material such as a film. The principle of this utilizes plasma resonance absorption of island particles. Particularly, it utilizes light absorption by surface plasma resonance occurring when light enters into the island particles of precious metals or transition metals and the absorption wavelength depends on the shapes of the particles and the permittivity therearound. Using elliptical-shaped island particles can differentiate the resonance wavelengths in major and minor axis directions, thereby achieving a polarizing property, in particular, they absorb polarized components parallel to the major axis and transmit polarized components parallel to the minor axis in long wavelength range. However, the POLARCOR, which only polarizes lights in the wavelength range near infrared region, does not cover visible light range required in liquid crystal displays. This is due to the physical property of silver used for the island particles.
Patent literature (PTL) 1 discloses a UV polarizing plate using silver as metal particles, wherein the particles are educed in glass by heat reduction to apply the above described principle. In this case, it is suggested that absorption in minor axis is utilized in contrast to the above mentioned POLARCOR. Although FIG. 1 of PTL 1 shows a function as a polarizing plate near 400 nm, the extinction ratio is small and the absorbable wavelength range is extremely narrow such that the polarizing plate cannot cover the entire visible light range even if POLARCOR and the technologies of PTL 1 are combined.
Non patent literature (NPL) 1 theoretically analyzes inorganic polarizing plates using plasma resonance of metal island particles. This literature describes that resonance wavelength of aluminum particles is approximately 200 nm shorter than that of silver particles, so that a polarizing plate covering visible light range can possibly be manufactured by using aluminum particles.
NPL 2 discloses some methods of manufacturing a polarizing plate by using aluminum particles. According to this literature, silicate-based glasses are not preferable for the substrate since aluminum reacts with the glasses and calcium aluminoborate glasses are suitable for this purpose (paragraphs 0018 and 0019). This limitation is at an economic disadvantage since the silicate-based glasses are widely spread as optical glasses and reliable products of them can be obtained at low cost. The literature also describes a method for forming island particles by etching a resist pattern (paragraphs 0037 and 0038). In general, a polarizing plate used in a projector is required to have a size of several centimeters and a high extinction ratio. Consequently, in the case of a polarizing plate for visible light, the resist pattern should have a size of several tens nanometers in order to be sufficiently shorter than visible light wavelengths and the high extinction ratio requires a high density pattern. Moreover, when used in a projector, a large area is required. In the described method of forming a high-density micro pattern by lithography, however, electron beam drawing technology and the like should be used to obtain the above-described pattern. The electron beam drawing technology, which draws respective patterns by electron beam, is not practical due to the low productivity thereof.
Although PTL 2 describes that aluminum are removed with chlorine plasma, in this case, chloride is likely to adhere to the aluminum pattern etched in such a manner. The chloride can be removed with a commercially available wet etchant (for example, SST-A2 available from Tokyo Ohka Kogyo Co., Ltd. in Japan); however, the etchant of this kind reacting with aluminum chloride also reacts with aluminum with a lower etching rate, making it difficult to form a desired pattern by using the described method.
In addition, PTL 2 also describes another method in which aluminum is deposited on a photo resist by glancing angle deposition (GLDA) and then the photo resist is removed (paragraphs 0045 and 0047). In this kind of method, it is contemplated that aluminum must be deposited also on the substrate surface in a certain extent to obtain an adhesion between the substrate and the aluminum. This means, however, the shape of the deposited aluminum film will differ from prolate sphere including prolate spheroid described as a suitable shape in paragraph 0015. As described in paragraph 0047, over-deposited materials are removed by anisotropic etching perpendicular to the surface. The shape anisotropy of aluminum is essential for the function of the polarizing plate. It therefore seems to be necessary that the amounts of aluminum deposited on resist part and substrate surface should be adjusted in order to obtain a desired shape by etching; however, controlling these in a size less than sub-micron size, such as 0.05 μm as described in paragraph 0047 is extremely difficult. Consequently, the productivity of this manufacturing method is suspicious. In addition, although the polarizing plate is required to have a property of a high transmittance in transmission axis direction, in the case of glass substrate, reflection of several percent from the glass interface is usually unavoidable, making it difficult to obtain the high transmittance.
PTL 3 discloses a method of forming a polarizing plate by glancing angle deposition (GLDA). This method, which obtains a polarizing property by forming micro-prismatic structures by glancing angle deposition of materials transparent and opaque to wavelengths in used bandwidth, seems to have a high productivity since micro patterns can be obtained by a process easier than the process explained in PTL 1. It should be noted that, the aspect ratio of the micro-prismatic structures of the material opaque to wavelengths in used bandwidth, the distance between each micro-prismatic structure and the linearity of the micro-prismatic structures are essential parameters for obtaining an excellent polarizing property and these parameters should be intentionally controlled also from a view point of property reproducibility; however, intentionally controlling these parameters is difficult since this method utilizes a phenomenon in which initial deposition layer of vapor particles blocks the subsequent vapor particles so that the prismatic structures are obtained by the lack of deposition of the vapor particles due to this shading. For dealing this problem, described is a method of providing polishing traces on a substrate by a rubbing process before the vapor deposition. Since the particle diameter of the vapored film is at most several tens nanometers, pitches less than sub-micron size should be intentionally produced by polishing in order to control the anisotropy of the particles. Unfortunately, ordinary polishing sheets, having a limitation of sub-microns, cannot easily produce such microscopic polishing traces. Furthermore, although the resonance wavelength of Al particles strongly depends on the refractive index therearound and how to combine the transparent and opaque materials is important in this case, no combination is disclosed in PTL 3 for obtaining an excellent polarizing property in visible light range. In addition, similarly to the case of PTL 1, when using a glass for the substrate, reflection of several percent from the glass interface is unavoidable.
NPL 2 discloses a laminated polarizing plate (“LAMIPOL”) for infrared communication. This has a laminated structure of Al and SiO2 and, according to this literature, has an extremely high extinction ratio. NPL 3 describes that using Ge instead of Al, which is light absorbing part of LAMIPOL, can achieve a high extinction ratio at wavelengths less than 1 μm. Te (tellurium) is also expected to achieve a high extinction ratio, according to FIG. 3 of the NPL 3. Although LAMIPOL is an absorbing typed polarizing plate achieving a high extinction ratio as described above, since the size of the light receiving surface is the lamination thickness of the light absorbing and transmitting materials, LAMIPOL is not suitable for use in projectors requiring the size of several centimeters square.
PTL 4 describes a polarizing plate combining a wire grid structure and an absorbing film. In the case of using a metal or semiconductor material as the absorbing film, the property of the film strongly depends on the optical property of the material such that reflectance in a particular range can be controlled by adjusting the thickness of a dielectric layer between the material and the wire grid. However, this adjustment is difficult to be performed in a wide wavelength range.
Although the bandwidth can be expanded by using Ta or Ge having a high absorption capability, this makes absorption in transmittance axis direction be larger, thus reducing transmittance in transmission axis direction, which is one of an important property of polarizing plates.
Application of fine particles to the absorption film has been proposed to solve the above-problem. However, conventional methods which directly deposit an absorbing film by using glancing angle deposition, which rely on self-organization by shadowing by the deposited absorbing film, are strongly affected by physical property of the material itself or roughness of the substrate, making it difficult to control the absorbing property.